THE MAGNETISM OF IRON. 91 



72. A very long steel ribbon of which the thickness is o. i cen- 

 timeter and the width is 30 centimeters is magnetized so that one 

 edge becomes a north pole and the other edge becomes a south 

 pole, as shown in Fig. 56, the intensity of magnetization being 

 800 units pole for each square centimeter of section of the steel 

 (80 units pole for each centimeter length of edge). Find the in- 

 tensity of the magnetic field due to the north polar edge of the 

 strip at a point distant 1 8 centimeters from the edge and specify 

 its direction. Ans. 8.89 gausses. 



73. Find the intensity of the resultant field at the point p in 

 Fig. 56, and determine the value of the angle 9, using the data 

 given in problem 72. Ans. //= ii.n gausses, 6 = 16 16'. 



74. One of the magnets specified in problem 90 is balanced 

 horizontally on a knife edge at Washington. The magnet weighs 

 1 20 grams. Find the horizontal distance from the knife edge to 

 the center of the bar taking the acceleration of gravity to be 980 

 centimeters per second per second. Use the data specified in 

 problem 66. Ans. 0.046 centimeter. 



75. The moment of inertia of one of the magnets specified in 

 problem 60 is 9,000 gr.-cm 2 . Calculate the time of one com- 

 plete oscillation of this magnet when it is suspended horizontally 

 at Washington. Ans. 11.15 seconds. 



76. A magnet makes one complete oscillation per second in a 

 magnetic field of which the intensity is 0.2 gauss. Another 

 magnet is twice as long, twice as wide, and twice as thick, it is 

 magnetized to twice the intensity (units pole per units sectional 

 area) and it is suspended in a field of which the intensity is o.i 

 gauss. What is its period of oscillation ? Ans. 2 seconds. 



Note. The moment of inertia of a rotating body is equal to the product of the 

 mass of the body into the square of its radius of gyration. Given two bodies of exactly 

 the same shape, their radii of gyration are proportional to their linear dimensions 

 whereas their masses are proportional to their volumes. 



77. A suspended magnet makes 20 oscillations in 184.5 sec ~ 

 onds at one place, and 20 oscillations in 21 5.8 seconds at another 



